The present invention relates to an electrode coated with an improved electron emission material for discharge lamp.
It has been known to use a mixture of barium oxide (BaO), calcium oxide (CaO) and yttrium oxide (Y.sub.2 O.sub.3) or a mixture of yttrium oxide and barium tungstate (Ba.sub.3 WO.sub.6) or barium-strontium-calcium tungstate (Ba.sub.2-x Sr.sub.x CaWO.sub.6) (x = 0 to 0.5) as the electron emission material coated on the electrode of the discharge lamp such as a high pressure metal-vapor lamp e.g. a high pressure mercury-vapor lamp.
However, the electron emission material containing yttrium oxide as heat-resistant oxide usually has a disadvantage of weak adhesion to the substrate of the electrode. During operation, a part of electron emission material is peeled off whereby it causes the sudden decrease of the lumen maintenance and the rise of the starting voltage to render the lamp inoperative.
In order to overcome the disadvantage, it has been proposed to improve the adhesion of the electron emission material on the substrate of the electrode by the addition of a small amount of silicon oxide (SiO.sub.2), zirconium oxide (ZrO.sub.2), aluminum oxide (Al.sub.2 O.sub.3), etc. The adhesion may be slightly improved by these addition, however, the cause of the short-life of the discharge lamp could not be substantially eliminated.